In a manufacturing process of semiconductor devices such as an integrated circuit (IC), a large-scale integration (LSI) circuit and the like, there has been used a semiconductor manufacturing apparatus for forming a semiconductor film on a wafer.
Referring to FIG. 2, there is illustrated a schematic diagram of a conventional semiconductor manufacturing apparatus.
The semiconductor manufacturing apparatus includes a heater 1, a reaction chamber 2 being heated by the heater 1 to a temperature appropriate for performing a semiconductor film forming process, a quartz boat 3 being taken into the reaction chamber 2 along the arrow direction with wafers loaded thereon, a wafer carrier 4 for transferring silicon wafers 14 between a wafer cassette 7 and the quartz boat 3, the wafer cassette 7 acting as a carrier bringing wafers into or out of the apparatus, a process gas line 9 through which a process gas for inducing the semiconductor film forming reaction is supplied to the apparatus, and a vacuum exhaust 10 for evacuating the reaction chamber 2 to vacuum or maintaining the inner pressure of the reaction chamber 2 at a certain level.
Referring to FIG. 3, there is schematically shown the reaction chamber 2 into which the quartz boat 3 is loaded during a semiconductor film forming process. A plurality of silicon wafers 14, only some of the wafers being depicted for illustration purpose, loaded on the quartz boat 3 are arranged in the reaction chamber 2 while maintaining a constant interval therebetween.
The process gas line 9 provides the reaction chamber 2 with the process gas, which induces the semiconductor film forming reaction while it passes through an internal tube 13 to thereby form semiconductor films on the silicon wafers 14. After the reaction process is completed, the used process gas is removed by the vacuum exhaust 10 through a space between an external tube 12 and the internal tube 13. In FIG. 3, one or more adiabatic plates 15 serve to reduce or prevent heat loss from the reaction chamber 2.
FIG. 4 describes a wafer arrangement during the semiconductor film forming process. In such a batch-type semiconductor film forming apparatus, the semiconductor film forming process should be carried out under a constant reaction condition so as to reproducibly obtain the thickness uniformity of the semiconductor films formed on silicon wafers. For this purpose, the wafers are arranged as illustrated in FIG. 4.
First of all, since the reaction condition near both ends of the wafer arrangement is not stabilized, dummy wafers are arranged in regions near the ends (indicated as an upper and a lower dummy wafer regions in FIG. 4), wherein a dummy wafer represents a wafer not being used for producing final products. In the remaining region of the wafer arrangement (designated as a process region in FIG. 4), the semiconductor films are formed under a controlled reaction condition on silicon wafers to be used for producing semiconductor products.
In case there are empty spaces between the silicon wafers positioned in the process region, the non-uniformity of the reaction condition which might be caused due to irregular intervals between the silicon wafers may be prevented by supplementing the empty spaces with dummy wafers as shown in FIG. 4.
As described above, the conventional batch-type semiconductor film forming apparatus achieves the uniformity of the reaction condition by using the dummy wafers. However, the use of dummy wafers leads to a waste, thereby increasing, especially, in a case of small batch production where various products in small quantities are produced, the semiconductor manufacturing cost.
The dummy wafers are loaded in a wafer cassette first and then transferred into the apparatus. Thereafter, a required number of dummy wafers are loaded on the quartz boat after being retrieved from the wafer cassette.
After being used for a predetermined number of times, the dummy wafers are taken out from the semiconductor manufacturing apparatus and then subject to a cleaning process thereof. The time required for the dummy wafer unloading process and the cleaning process also gives rise to an increase in the semiconductor manufacturing time.